Permissive licenses are also known as BSD-style or Apache-style licenses.
They contain minimal requirements about how the software can be modified or redistributed, clauses that are not existent in public domain or public domain equivalent licenses..
Permissive licenses are more adequate than restrictive licenses when ensuring that code can be shared freely, is reusable and is compatible with other licenses.
On the other hand, its source code is more prone to be used in commercial or patented projects compared to restrictive licenses.
Example of permissive licenses are:
- Boost Software License 1.0
- ISC license
- MIT licenses
- BSD 2-clause “simplified” license
- BSD 3-clause “new” or “revised” license
- Apache license 2.0
The most popular permissive licenses are MIT and Apache 2.0
ISC
Internet System Consortium (ISC) license is a permissive license, not as popular as MIT license or BSD.
Some users prefer it to MIT before it shortness, and software like npm proposes it as default license for new projects.
Some organizations like Joinup.eu recommends to use the more popular MIT license instead of ISC to reduce license proliferation.
MIT license
MIT license is license developed in the MIT university.
It is probably the most popular permissive license. As it is a very short license, it ensures re-usability and license compatibility, though it is not the most protective for developers.
FSF warns that MIT does not provide patent grant, and recommends using Apache 2.0 license instead for those interested in permissive licenses.
The FSF recommends to refer to it as X11 license, as according to them there is more than a license issued by MIT, for example the Expat license.
On the other hand, mainstream users and organizations like the Open Source Iniciative or Joinup.eu use the term “MIT” license, and Wikipedia considers that both X11 and Expat are variants of the original MIT license. This post refers to this license as MIT license.
MIT license is compatible with GNU GPL licenses.
BSD 2-clause license
BSD 2-clause license is similar to MIT license.
BSD 3-clause license
BSD 3-clause license is more restrictive than BSD 2-clause license.
As of 2023, the 3-clause is the most frequent of the BSD clauses. It is derived from the original BSD 4-clause license.
Apache license, version 2.0
Apache license, version 2.0 is among the most popular permissive licenses.
On the other hand, it is not the most reusable and compatible. For example, according to the FSF, Apache 2.0 license is not compatible with GPLv2 (unlike MIT), meaning that Apache 2.0 software cannot be sublicensed as GPLv2.
Apache 2.0 is compatible with GPLv3 license, but it is not compatible with GPLv2. The reason is that Apache 2.0 is more .restrictive regarding patent grant than GPLv2, but GPLv3 was made equally restrictive.
FSF recommends the Apache license over the other permissive licenses because it prevents “patent treachery”. You can read this opinion on this external link. With the term “patent treachery”, FSF refers to the practice of contributors or third-party suing for patent infringement, and Apache adds some clauses to hinder it.
You can read more about Apache license, version 2.0 on this post.
Artistic License 2.0
Artistic License 2.0 was originally used for Perl programming language.
It is not as popular as other permissive licenses like MIT and Apache license 2.0.